In the amplification and broadcast of music or other performances, either live or from recordings, the tonal content of the broadcast audio program can be distorted by frequency dependent attenuation or reinforcement from characteristics of the room, concert hall, speaker system, or other factors affecting the sound. An equalizer is often used in the amplification system to correct or produce a desired frequency response of the broadcast system and environment producing the audio program heard by the audience. In an ideal mathematical sense, the broadcast system and environment can be considered as a transfer function, H(s), and the desired frequency response can be considered as a function, H.sub.d (s). If the system is first brought to unity, i.e., 1=H(s).times.H.sup.-1 (s), and followed by H.sub.d (s), the overall series of transfer functions is: H(s).times.H.sup.-1 (s).times.H.sub.d (s), which is equal to the desired transfer function H.sub.d (s). The series functions H.sup.-1 (s).times.H.sub.d (s) are equal to a correction function H.sub.c (s) which when applied to the transfer function H(s) yields the desired transfer function, i.e., H.sub.d (s)=H(s).times.H.sub.c (s).
In many audio amplification systems, the correction function, H.sub.c (s), is approximated by dividing the audio spectrum into a plurality of frequency bands and then selectively adjusting the attenuation, or amplification, of each of the corresponding frequency bands to produce the desired frequency response. In a system with a manual equalizer, a reference sound consisting of "white noise" which has a constant amplitude vs. frequency characteristic across the audio spectrum, or "pink noise" which has a frequency spectrum content equal on a logarithmic basis across the audio spectrum, is applied to the input of the system, and the broadcast noise is picked up by a microphone connected to an audio spectrum analyzer. The operator then adjusts the gain of the frequency bands, such as by slide attenuators of the equalizer, until a desired or flat frequency response appears on the audio spectrum analyzer. The prior art also discloses automatic systems using pink or white noise with frequency analyzing units providing control signals to automatically set equalizers. Manual or automatic set up of an equalizer using pink or white noise during a performance is not desirable because the noise, which roars like Niagara Falls, would be objectionable to most audiences. Thus, the equalizer systems using pink or white noise cannot compensate for changes in the acoustics of the performance area such as caused by the presence of an audience, movement of microphones, etc., during the performance.
The prior art also contains automatic equalizers which simultaneously monitor the input signal and the audio output of an audio amplification system. The audio output broadcast by speakers is monitored by a microphone placed in the listening environment to produce a reference signal. The magnitudes of frequency bands in the input signal are compared to the corresponding magnitudes of frequency bands in the reference signal. The outputs of the comparitors are used to automatically adjust an equalizer. The equalizer can be in the form of a digital filter such as a finite impulse response (FIR) or an infinite impulse response (IIR) digital filter practiced by a corresponding algorithm in a microprocessor such as a digital signal processor (DSP). Inherent delays between the input signal and the reference signal pickup as well as rapid variation in frequency content and magnitude of the incoming program signal, result in errors in determining the present frequency response of the broadcast system causing distortion in the equalized program.